Magmatic Processes in the Alta Stock, Utah: The Impact of Tectonic Dilation on Granite Differentiation

Emmaline Saunders¹, Michael Stearns¹, John M. Bartley², John R. Bowman²
1. Utah Valley University, Orem, Utah; 2. University of Utah, Salt Lake City, Utah

Helpful Definitions for THIS Project

Pluton - Simplest terms, an ancient magma chamber

Stock - The feeder that transfers the magma to be erupted

Textures - Rock distinctions

Equigranular - Equal grains distributed equally

Porphyritic - Not equal grain distribution (opposite of equigranular)

Mafic - A chemical distinction in the rocks (the dark colored minerals)

Felsic - The other end of the chemical distinction in the rocks (the light colored minerals)

Emplaced - A term for magma accumulation in the pluton

Plagioclase, Quartz and Orthoclase - Minerals found in the rocks that can be clasified as mafic or felsic

BSE (Back Scatter Electron) - A type of imaging on an SEM that deals with the average atomic numbers of chemicals

EBSD (Electron Back Scatter Diffraction) - A type of analysis on an SEM that looks at small tiny deformations in the atomic structure of a crystal

Introduction & Background

The Alta stock, UT, is part of the Wasatch Intrusive Belt and most well-known for its surrounding metamorphic aureole. Baker (1965) defined two textural units within the Alta stock: an equigranular, more mafic border phase and a porphyritic, more felsic central phase. The contacts between the two phases varies from sharp to gradational. Zircon and titanite U-Pb dates range from ~35–31 Ma and young inward suggesting the margins of the stock were emplaced before the central phase.

Plagioclase (Plg), quartz and orthoclase (Qtz+Or) mineral modes from 40 Alta stock samples show a negatively sloped linear trend (R2 = 0.86) between equigranular samples with high Plg/low Qtz+Or samples and and an aplitic sample with low Plg/high Qtz+Or.

Methods & Results

Porphyritic samples lie between the two end members along the linear trend. Plagioclase within the equigranular samples form an interlocking framework with abundant Plg-to-Plg grain boundaries.
Crystal plastic deformation (e.g., pressure solution, undulose extinction, mechanical twinning of Plg, and kink bands in biotite) of early-crystallizing phases is more common within the equigranular samples.
Plg in the porphyritic samples are more often solitary phenocrysts surrounded by a matrix of fine-grained Qtz+Or, and have fewer Plg-to-Plg contacts.

Discussion

The steep wall rock contacts, correlated patterns of textural and lithologic variation, and zircon and titanite U-Pb petrochronology are best explained by protracted, tectonic dilation (~0.5 mm a-1) of a dike-like conduit that likely transmitted magma to the surface and accumulated magmatic rock. The decreasing amount of Plg and of interlocking Plg-Plg grain boundaries toward the center of the stock are interpreted to have resulted from tectonic forces that drove aplitic melt from the partially molten equigranular border phase into the higher melt percent porphyritic central phase rather than a sidewall or gravitational fractionation process.

Conclusions

The crystal plastic deformation of a plagioclase framework present in the equigranular border phase samples suggests this melt moved after rheological lock up of the magmas. These relationships provides insights into how magmas may differentiate at the emplacement level and highlights the interplay between tectonic forces and petrologic processes within growing intrusive bodies.
Future research may include: